1. Field of the Invention
The present invention relates to a method for manufacturing a rare earth magnet and a powder compacting apparatus used in the manufacturing method.
2. Discussion of the Related Art
A rare earth sintered magnet is produced by pulverizing an alloy for a rare earth magnet to form alloy powder, compacting the alloy powder, and subjecting the alloy powder to a sintering process and an aging process. Currently, there are two kinds of magnets known as the rare earth sintered magnets, i.e., a samarium-cobalt magnet and a neo-dymium-iron-boron magnet each of which are widely used in various fields. Hereinafter, the magnet of neodymium, iron, and boron system is referred to as an “R-T-(M)-B type magnet”, where R represents a rare earth element or yttrium, T represents iron, or a transition metal in which cobalt or nickel is substituted for part of iron, M represents an additional element, and B represents boron or a compound of boron and carbon. Between the two kinds of magnets, the R-T-(M)-B type magnet exhibits the maximum magnetic energy product among various kinds of magnets, and the price thereof is relatively cheap. For these reasons, the R-T-(M)-B type magnet is used for various kinds of electronics appliances.
When an anisotropic rare earth sintered magnet is manufactured, an orienting magnetic field is applied to magnetic powder during press compaction. Thus, the produced compact is in a strongly magnetized condition. In order to remove the magnetization, a demagnetizing process is performed in a press, however, it is extremely difficult to attain the perfect demagnetization. Therefore, when the demagnetized compact is ejected from a die hole (a cavity) of a press, magnetic powder, which is dispersed around the die hole, is strongly attracted to the compact According to measurements, a magnetization of 0.002 to 0.006. T (tesla) remains in the compact after the demagnetizing process.
Since the demagnetizing process is performed for the compact while in the cavity, the intensity variation of the magnetic field formed for the demagnetization process is designed so as to have the most suitable profile for the demagnetization of the compact in the center portion of the cavity. As a result, magnetic powder adhering to magnetic material components of a magnetic field generating portions positioned over and under the cavity and magnetic powder adhering on the die of the press and the like are demagnetized only a little. According to measurements, a magnetization of about 0.005 to 0.010 T remains in the powder adhering to a pole piece (a magnetic portion of an upper punch) accompanied with the magnetic field generating coil.
The compact and powder, both of which are magnetized, mutually attract each other strongly. Accordingly, when the compact is ejected from the cavity of the press and placed onto a carrying device, the magnetic powder adhering to the upper punch of the pressing apparatus and the magnetic powder scattered on the die are attracted to the compact, and firmly adsorbed to the surface of the compact.
In order to remove the magnetic powder adhering to the surface of the compact from the surface of the compact, nitrogen gas (N2 gas) was sprayed on the compact while the compact is being carried on a carrying belt and transported.
However, it is impossible to entirely remove the magnetic powder adhering to a portion of the compact which little receives the N2 gas. Therefore, the magnetic powder attracted to the surface of the compact by the strong magnetic force, results in the remaining magnetic powder being welded to the surface of a sintered compact body by sintering. This magnetic powder, welded by the sintering, increases the degree of unevenness in the surface of the sintered compact body. Thus, it is necessary to remove the welded portions by grinding to provide a smooth surface on the sintered body.
Conventionally, after the large block-like sintered, compact body was produced, the body was processed by cutting, so as to obtain a plurality of relatively small sintered bodies. In this instance, even if protrusions caused by the adhering powder existed on the surface of the sintered compact body, the protrusions in the surface of the respective sintered bodies cut out by the cutting process did not cause serious problems.
In order to improve the production yield of small magnets, however, a pressing process has been recently adopted in which the compact produced has the shape of the final product. In this instance, if the undesired magnetic powder adheres to the surface of the compact produced, the period of time to complete the grinding process after the sintering is increased, and the advantages of mass production are diminished.
Japanese Laid-Open Patent Publication No. 3-234603 discloses a powder removing device in which a ceramic powder compact situated in a cylindrical brush, and the powder adhering to the surface of the compact is blown off while the brush is rotating.
If these techniques are adapted to the production of a compact from rare earth magnetic powder, the following problems arise.
A compact of a rare earth alloy powder, in which the powder orientation in the magnetic field is significant, has a compact density that is suppressed to be as low as a density of 3.9 to 5.0 g/cm3, which is soft. Further, in the case where the rare earth alloy powder is produced by a rapid cooling method, the particle size distribution curve of the powder is sharp. Thus, the strength of the rare earth compact is lowered when compared with the strength of a compact using powder produced by an ingot casting method. Additionally, if the surface of the compact is rubbed with a brush, the corners of the compact may be lost, or the compact may be broken.
It takes time and effort to insert the rare earth compact into a powder removing device and to take it out of the powder removing device, such that the overall production yield is reduced.
An additional disadvantage, is that the recovered powder reacts with oxygen in the air, so as to be rapidly oxidized. Thus, the possibility exists that a burning accident may occur in the powder removing device which is a dangerous situation.
For the reasons described above, an optimum powder removing device is required in a method for manufacturing a rare earth sintered magnet.